This invention relates to an improvement in a high heat-conductive circuit board which can be suitably used for high speed signal propagation, and so on in a high frequency region, particularly for multilayer circuits.
With development and progress in miniaturization, etc. of electronic equipments in recent years, packaging density of those electrical elements such as integrated circuits (IC), etc. on the circuit board has become increased. Further, considering that power semiconductors, etc. are to be mounted on such circuit board, amount of heat generated on such board has (and will) become much greater.
On the other hand, looking into the materials for the substrate of the circuit boards, it is found that the ceramic substrate is more heat-resistant than the resin substrate. Of those various ceramic substrates, the Al.sub.2 O.sub.3 substrate is cheap in price, and is excellent in its mechanical strength and electrical insulation, besides which it can be readily prepared into green sheets. On account of such advantages it possesses, Al.sub.2 O.sub.3 has been widely used for fabrication of high density circuit board such as multi-layered interconnection, and so forth. However, since this material has its heat-conductivity as low as 20 W/m.multidot.k or so, development of other appropriate material has been longed for.
In answer to this desire, there have been developed various materials such as SiC, AlN, and others which are excellent in their mechanical strength. Since these materials are also excellent in the heat-conductivity, there were attempts to use them as the circuit substrate. However, SiC is inadequate to be used as the substrate, on which those electrical elements to be applied with high electric voltage are mounted, because of its high dielectric constant and low dielectric strength. As an alternative, AlN has drawn attention as the material for the circuit substrate, and one embodimental construction is proposed in, for example, Japanese Patent Kokai Publication No. 61-119094. According to this Kokai Publication, it is disclosed that formation of an oxidized layer on the surface of the AlN substrate enables metallized conductors (made of thick film paste) thereon by heating. As the methods for forming such oxidized layer, there are proposed three manners: (1) treating the AlN substrate in an atmosphere at a temperature of 1,250.degree. C. for one hour; (2) treating it by immersion in phosphoric acid liquid; and (3) leaving it in water vapor at a temperature of 121.degree. C. and under a pressure of 2 atm. for 168 hours.
The above-described conventional technique has drawbacks that the substrate material is limited to AlN; an ordinary vitreous (or glassy) material is used for the resistor paste and a lead frame is used for the circuit conductor, wherein, however, printing of Ag-Pd paste is directly applied on to the substrate, hence microcracks or cracks, etc. tend to occur readily entailing difficulties in constructing the multilayered circuit board; and further, formation of the oxidized layer is limited to the abovementioned three manners.